Network Working Group R. Braden, Ed.
Request for Comments: 2205 ISI
Category: Standards Track L. Zhang
UCLA
S. Berson
ISI
S. Herzog
IBM Research
S. Jamin
Univ. of Michigan
September 1997
Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This memo describes version 1 of RSVP, a resource reservation setup
protocol designed for an integrated services Internet. RSVP provides
receiver-initiated setup of resource reservations for multicast or
unicast data flows, with good scaling and robustness properties.
Braden, Ed., et. al. Standards Track [Page 1]
RFC 2205 RSVP September 1997
Table of Contents
1. Introduction ................................................... 4
1.1 Data Flows ................................................. 7
1.2 Reservation Model .......................................... 8
1.3 Reservation Styles .........................................11
1.4 Examples of Styles .........................................14
2. RSVP Protocol Mechanisms .......................................19
2.1 RSVP Messages ..............................................19
2.2 Merging Flowspecs ..........................................21
2.3 Soft State .................................................22
2.4 Teardown ...................................................24
2.5 Errors .....................................................25
2.6 Confirmation ...............................................27
2.7 Policy Control .............................................27
2.8 Security ...................................................28
2.9 Non-RSVP Clouds ............................................29
2.10 Host Model ................................................30
3. RSVP Functional Specification ..................................32
3.1 RSVP Message Formats .......................................32
3.2 Port Usage .................................................47
3.3 Sending RSVP Messages ......................................48
3.4 Avoiding RSVP Message Loops ................................50
3.5 Blockade State .............................................54
3.6 Local Repair ...............................................56
3.7 Time Parameters ............................................57
3.8 Traffic Policing and Non-Integrated Service Hops ...........58
3.9 Multihomed Hosts ...........................................59
3.10 Future Compatibility ......................................61
3.11 RSVP Interfaces ...........................................63
4. Acknowledgments ................................................76
APPENDIX A. Object Definitions ....................................77
APPENDIX B. Error Codes and Values ................................92
APPENDIX C. UDP Encapsulation .....................................98
APPENDIX D. Glossary .............................................102
REFERENCES .......................................................111
SECURITY CONSIDERATIONS ..........................................111
AUTHORS' ADDRESSES ...............................................112
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What's Changed
This revision contains the following very minor changes from the ID14
version.
o For clarity, each message type is now defined separately in
Section 3.1.
o We added more precise and complete rules for accepting Path
messages for unicast and multicast destinations (Section
3.1.3).
o We added more precise and complete rules for processing and
forwarding PathTear messages (Section 3.1.5).
o A note was added that a SCOPE object will be ignored if it
appears in a ResvTear message (Section 3.1.6).
o A note was added that a SENDER_TSPEC or ADSPEC object will be
ignored if it appears in a PathTear message (Section 3.1.5).
o The obsolete error code Ambiguous Filter Spec (09) was
removed, and a new (and more consistent) name was given to
error code 08 (Appendix B).
o In the generic interface to traffic control, the Adspec was
added as a parameter to the AddFlow and ModFlow calls
(3.11.2). This is needed to accommodate a node that updates
the slack term (S) of Guaranteed service.
o An error subtype was added for an Adspec error (Appendix B).
o Additional explanation was added for handling a CONFIRM
object (Section 3.1.4).
o The rules for forwarding objects with unknown class type were
clarified.
o Additional discussion was added to the Introduction and to
Section 3.11.2 about the relationship of RSVP to the link
layer. (Section 3.10).
o Section 2.7 on Policy and Security was split into two
sections, and some additional discussion of security was
included.
o There were some minor editorial improvements.
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1. Introduction
This document defines RSVP, a resource reservation setup protocol
designed for an integrated services Internet [RSVP93, RFC 1633]. The
RSVP protocol is used by a host to request specific qualities of
service from the network for particular application data streams or
flows. RSVP is also used by routers to deliver quality-of-service
(QoS) requests to all nodes along the path(s) of the flows and to
establish and maintain state to provide the requested service. RSVP
requests will generally result in resources being reserved in each
node along the data path.
RSVP requests resources for simplex flows, i.e., it requests
resources in only one direction. Therefore, RSVP treats a sender as
logically distinct from a receiver, although the same application
process may act as both a sender and a receiver at the same time.
RSVP operates on top of IPv4 or IPv6, occupying the place of a
transport protocol in the protocol stack. However, RSVP does not
transport application data but is rather an Internet control
protocol, like ICMP, IGMP, or routing protocols. Like the
implementations of routing and management protocols, an
implementation of RSVP will typically execute in the background, not
in the data forwarding path, as shown in Figure 1.
RSVP is not itself a routing protocol; RSVP is designed to operate
with current and future unicast and multicast routing protocols. An
RSVP process consults the local routing database(s) to obtain routes.
In the multicast case, for example, a host sends IGMP messages to
join a multicast group and then sends RSVP messages to reserve
resources along the delivery path(s) of that group. Routing
protocols determine where packets get forwarded; RSVP is only
concerned with the QoS of those packets that are forwarded in
accordance with routing.
In order to efficiently accommodate large groups, dynamic group
membership, and heterogeneous receiver requirements, RSVP makes
receivers responsible for requesting a specific QoS [RSVP93]. A QoS
request from a receiver host application is passed to the local RSVP
process. The RSVP protocol then carries the request to all the nodes
(routers and hosts) along the reverse data path(s) to the data
source(s), but only as far as the router where the receiver's data
path joins the multicast distribution tree. As a result, RSVP's
reservation overhead is in general logarithmic rather than linear in
the number of receivers.
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RFC 2205 RSVP September 1997
HOST ROUTER
_____________________________ ____________________________
| _______ | | |
| | | _______ | | _______ |
| |Appli- | | | |RSVP | | | |
| | cation| | RSVP RSVP
| | | | | _______ | | |
| | | |process| _____ | ||Routing| |process| _____ |
| |_._____| | -->Polcy|| || -->Polcy||
| | |__.__._| |Cntrl|| ||process| |__.__._| |Cntrl||
| |data | | |_____|| ||__.____| | | |_____||
|===|===========|==|==========| |===|==========|==|==========|
| | --------| | _____ | | | --------| | _____ |
| | | | ---->Admis|| | | | | ---->Admis||
| _V__V_ ___V____ |Cntrl|| | _V__V_ __V_____ |Cntrl||
| | | | | |_____|| | | | | ||_____||
| |Class-| | Packet | | | |Class-| | Packet | |
| | ifier|==>Schedulr|================> ifier|==>Schedulr|===========>
| |______| |________| |data | |______| |________| |data
| | | |
|_____________________________| |____________________________|
Figure 1: RSVP in Hosts and Routers
Quality of service is implemented for a particular data flow by
mechanisms collectively called "traffic control". These mechanisms
include (1) a packet classifier, (2) admission control, and (3) a
"packet scheduler" or some other link-layer-dependent mechanism to
determine when particular packets are forwarded. The "packet
classifier" determines the QoS class (and perhaps the route) for each
packet. For each outgoing interface, the "packet scheduler" or other
link-layer-dependent mechanism achieves the promised QoS. Traffic
control implements QoS service models defined by the Integrated
Services Working Group.
During reservation setup, an RSVP QoS request is passed to two local
decision modules, "admission control" and "policy control".
Admission control determines whether the node has sufficient
available resources to supply the requested QoS. Policy control
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determines whether the user has administrative permission to make the
reservation. If both checks succeed, parameters are set in the
packet classifier and in the link layer interface (e.g., in the
packet scheduler) to obtain the desired QoS. If either check fails,
the RSVP program returns an error notification to the application
process that originated the request.
RSVP protocol mechanisms provide a general facility for creating and
maintaining distributed reservation state across a mesh of multicast
or unicast delivery paths. RSVP itself transfers and manipulates QoS
and policy control parameters as opaque data, passing them to the
appropriate traffic control and policy control modules for
interpretation. The structure and contents of the QoS parameters are
documented in specifications developed by the Integrated Services
Working Group; see [RFC 2210]. The structure and contents of the
policy parameters are under development.
Since the membership of a large multicast group and the resulting
multicast tree topology are likely to change with time, the RSVP
design assumes that state for RSVP and traffic control state is to be
built and destroyed incrementally in routers and hosts. For this
purpose, RSVP establishes "soft" state; that is, RSVP sends periodic
refresh messages to maintain the state along the reserved path(s).
In the absence of refresh messages, the state automatically times out
and is deleted.
In summary, RSVP has the following attributes:
o RSVP makes resource reservations for both unicast and many-to-
many multicast applications, adapting dynamically to changing
group membership as well as to changing routes.
o RSVP is simplex, i.e., it makes reservations for unidirectional
data flows.
o RSVP is receiver-oriented, i.e., the receiver of a data flow
initiates and maintains the resource reservation used for that
flow.
o RSVP maintains "soft" state in routers and hosts, providing
graceful support for dynamic membership changes and automatic
adaptation to routing changes.
o RSVP is not a routing protocol but depends upon present and
future routing protocols.
o RSVP transports and maintains traffic control and policy control
parameters that are opaque to RSVP.
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RFC 2205 RSVP September 1997
o RSVP provides several reservation models or "styles" (defined
below) to fit a variety of applications.
o RSVP provides transparent operation through routers that do not
support it.
o RSVP supports both IPv4 and IPv6.
Further discussion on the objectives and general justification for
RSVP design are presented in [RSVP93] and [RFC 1633].
The remainder of this section describes the RSVP reservation
services. Section 2 presents an overview of the RSVP protocol
mechanisms. Section 3 contains the functional specification of RSVP,
while Section 4 presents explicit message processing rules. Appendix
A defines the variable-length typed data objects used in the RSVP
protocol. Appendix B defines error codes and values. Appendix C
defines a UDP encapsulation of RSVP messages, for hosts whose
operating systems provide inadequate raw network I/O support.
1.1 Data Flows
RSVP defines a "session" to be a data flow with a particular
destination and transport-layer protocol. RSVP treats each
session independently, and this document often omits the implied
qualification "for the same session".
An RSVP session is defined by the triple: (DestAddress, ProtocolId
[, DstPort]). Here DestAddress, the IP destination address of the
data packets, may be a unicast or multicast address. ProtocolId
is the IP protocol ID. The optional DstPort parameter is a
"generalized destination port", i.e., some further demultiplexing
point in the transport or application protocol layer. DstPort
could be defined by a UDP/TCP destination port field, by an
equivalent field in another transport protocol, or by some
application-specific information.
Although the RSVP protocol is designed to be easily extensible for
greater generality, the basic protocol documented here supports
only UDP/TCP ports as generalized ports. Note that it is not
strictly necessary to include DstPort in the session definition
when DestAddress is multicast, since different sessions can always
have different multicast addresses. However, DstPort is necessary
to allow more than one unicast session addressed to the same
receiver host.
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Figure 2 illustrates the flow of data packets in a single RSVP
session, assuming multicast data distribution. The arrows
indicate data flowing from senders S1 and S2 to receivers R1, R2,
and R3, and the cloud represents the distribution mesh created by
multicast routing. Multicast distribution forwards a copy of each
data packet from a sender Si to every receiver Rj; a unicast
distribution session has a single receiver R. Each sender Si may
be running in a unique Internet host, or a single host may contain
multiple senders distinguished by "generalized source ports".
Senders Receivers
_____________________
( ) ===> R1
S1 ===> ( Multicast )
( ) ===> R2
( distribution )
S2 ===> ( )
( by Internet ) ===> R3
(_____________________)
Figure 2: Multicast Distribution Session
For unicast transmission, there will be a single destination host
but there may be multiple senders; RSVP can set up reservations
for multipoint-to-single-point transmission.
1.2 Reservation Model
An elementary RSVP reservation request consists of a "flowspec"
together with a "filter spec"; this pair is called a "flow
descriptor". The flowspec specifies a desired QoS. The filter
spec, together with a session specification, defines the set of
data packets -- the "flow" -- to receive the QoS defined by the
flowspec. The flowspec is used to set parameters in the node's
packet scheduler or other link layer mechanism, while the filter
spec is used to set parameters in the packet classifier. Data
packets that are addressed to a particular session but do not
match any of the filter specs for that session are handled as
best-effort traffic.
The flowspec in a reservation request will generally include a
service class and two sets of numeric parameters: (1) an "Rspec"
(R for `reserve') that defines the desired QoS, and (2) a "Tspec"
(T for `traffic') that describes the data flow. The formats and
contents of Tspecs and Rspecs are determined by the integrated
service models [RFC 2210] and are generally opaque to RSVP.
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The exact format of a filter spec depends upon whether IPv4 or
IPv6 is in use; see Appendix A. In the most general approach
[RSVP93], filter specs may select arbitrary subsets of the packets
in a given session. Such subsets might be defined in terms of
senders (i.e., sender IP address and generalized source port), in
terms of a higher-level protocol, or generally in terms of any
fields in any protocol headers in the packet. For example, filter
specs might be used to select different subflows of a
hierarchically-encoded video stream by selecting on fields in an
application-layer header. In the interest of simplicity (and to
minimize layer violation), the basic filter spec format defined in
the present RSVP specification has a very restricted form: sender
IP address and optionally the UDP/TCP port number SrcPort.
Because the UDP/TCP port numbers are used for packet
classification, each router must be able to examine these fields.
This raises three potential problems.
1. It is necessary to avoid IP fragmentation of a data flow for
which a resource reservation is desired.
Document [RFC 2210] specifies a procedure for applications
using RSVP facilities to compute the minimum MTU over a
multicast tree and return the result to the senders.
2. IPv6 inserts a variable number of variable-length Internet-
layer headers before the transport header, increasing the
difficulty and cost of packet classification for QoS.
Efficient classification of IPv6 data packets could be
obtained using the Flow Label field of the IPv6 header. The
details will be provided in the future.
3. IP-level Security, under either IPv4 or IPv6, may encrypt the
entire transport header, hiding the port numbers of data
packets from intermediate routers.
A small extension to RSVP for IP Security under IPv4 and IPv6
is described separately in [RFC 2207].
RSVP messages carrying reservation requests originate at receivers
and are passed upstream towards the sender(s). Note: in this
document, we define the directional terms "upstream" vs.
"downstream", "previous hop" vs. "next hop", and "incoming
interface" vs "outgoing interface" with respect to the direction
of data flow.
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RFC 2205 RSVP September 1997
At each intermediate node, a reservation request triggers two
general actions, as follows:
1. Make a reservation on a link
The RSVP process passes the request to admission control and
policy control. If either test fails, the reservation is
rejected and the RSVP process returns an error message to the
appropriate receiver(s). If both succeed, the node sets the
packet classifier to select the data packets defined by the
filter spec, and it interacts with the appropriate link layer
to obtain the desired QoS defined by the flowspec.
The detailed rules for satisfying an RSVP QoS request depend
upon the particular link layer technology in use on each
interface. Specifications are under development in the ISSLL
Working Group for mapping reservation requests into popular
link layer technologies. For a simple leased line, the
desired QoS will be obtained from the packet scheduler in the
link layer driver, for example. If the link-layer technology
implements its own QoS management capability, then RSVP must
negotiate with the link layer to obtain the requested QoS.
Note that the action to control QoS occurs at the place where
the data enters the link-layer medium, i.e., at the upstream
end of the logical or physical link, although an RSVP
reservation request originates from receiver(s) downstream.
2. Forward the request upstream
A reservation request is propagated upstream towards the
appropriate senders. The set of sender hosts to which a
given reservation request is propagated is called the "scope"
of that request.
The reservation request that a node forwards upstream may
differ from the request that it received from downstream, for
two reasons. The traffic control mechanism may modify the
flowspec hop-by-hop. More importantly, reservations from
different downstream branches of the multicast tree(s) from
the same sender (or set of senders) must be " merged" as
reservations travel upstream.
When a receiver originates a reservation request, it can also
request a confirmation message to indicate that its request was
(probably) installed in the network. A successful reservation
request propagates upstream along the multicast tree until it
reaches a point where an existing reservation is equal or greater
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RFC 2205 RSVP September 1997
than that being requested. At that point, the arriving request is
merged with the reservation in place and need not be forwarded
further; the node may then send a reservation confirmation message
back to the receiver. Note that the receipt of a confirmation is
only a high-probability indication, not a guarantee, that the
requested service is in place all the way to the sender(s), as
explained in Section 2.6.
The basic RSVP reservation model is "one pass": a receiver sends a
reservation request upstream, and each node in the path either
accepts or rejects the request. This scheme provides no easy way
for a receiver to find out the resulting end-to-end service.
Therefore, RSVP supports an enhancement to one-pass service known
as "One Pass With Advertising" (OPWA) [OPWA95]. With OPWA, RSVP
control packets are sent downstream, following the data paths, to
gather information that may be used to predict the end-to-end QoS.
The results ("advertisements") are delivered by RSVP to the
receiver hosts and perhaps to the receiver applications. The
advertisements may then be used by the receiver to construct, or
to dynamically adjust, an appropriate reservation request.
1.3 Reservation Styles
A reservation request includes a set of options that are
collectively called the reservation "style".
One reservation option concerns the treatment of reservations for
different senders within the same session: establish a "distinct"
reservation for each upstream sender, or else make a single
reservation that is "shared" among all packets of selected
senders.
Another reservation option controls the selection of senders; it
may be an "explicit" list of all selected senders, or a "wildcard"
that implicitly selects all the senders to the session. In an
explicit sender-selection reservation, each filter spec must match
exactly one sender, while in a wildcard sender-selection no filter
spec is needed.
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RFC 2205 RSVP September 1997
Sender || Reservations:
Selection || Distinct | Shared
_________||__________________|____________________
|| | |
Explicit || Fixed-Filter | Shared-Explicit |
|| (FF) style | (SE) Style |
__________||__________________|____________________|
|| | |
Wildcard || (None defined) | Wildcard-Filter |
|| | (WF) Style |
__________||__________________|____________________|
Figure 3: Reservation Attributes and Styles
The following styles are currently defined (see Figure 3):
o Wildcard-Filter (WF) Style
The WF style implies the options: "shared" reservation and
"wildcard" sender selection. Thus, a WF-style reservation
creates a single reservation shared by flows from all
upstream senders. This reservation may be thought of as a
shared "pipe", whose "size" is the largest of the resource
requests from all receivers, independent of the number of
senders using it. A WF-style reservation is propagated
upstream towards all sender hosts, and it automatically
extends to new senders as they appear.
Symbolically, we can represent a WF-style reservation request
by:
WF( * {Q})
where the asterisk represents wildcard sender selection and Q
represents the flowspec.
o Fixed-Filter (FF) Style
The FF style implies the options: "distinct" reservations and
"explicit" sender selection. Thus, an elementary FF-style
reservation request creates a distinct reservation for data
packets from a particular sender, not sharing them with other
senders' packets for the same session.
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RFC 2205 RSVP September 1997
Symbolically, we can represent an elementary FF reservation
request by:
FF( S{Q})
where S is the selected sender and Q is the corresponding
flowspec; the pair forms a flow descriptor. RSVP allows
multiple elementary FF-style reservations to be requested at
the same time, using a list of flow descriptors:
FF( S1{Q1}, S2{Q2}, ...)
The total reservation on a link for a given session is the
`sum' of Q1, Q2, ... for all requested senders.
o Shared Explicit (SE) Style
The SE style implies the options: "shared" reservation and
"explicit" sender selection. Thus, an SE-style reservation
creates a single reservation shared by selected upstream
senders. Unlike the WF style, the SE style allows a receiver
to explicitly specify the set of senders to be included.
We can represent an SE reservation request containing a
flowspec Q and a list of senders S1, S2, ... by:
SE( (S1,S2,...){Q} )
Shared reservations, created by WF and SE styles, are appropriate
for those multicast applications in which multiple data sources
are unlikely to transmit simultaneously. Packetized audio is an
example of an application suitable for shared reservations; since
a limited number of people talk at once, each receiver might issue
a WF or SE reservation request for twice the bandwidth required
for one sender (to allow some over-speaking). On the other hand,
the FF style, which creates distinct reservations for the flows
from different senders, is appropriate for video signals.
The RSVP rules disallow merging of shared reservations with
distinct reservations, since these modes are fundamentally
incompatible. They also disallow merging explicit sender
selection with wildcard sender selection, since this might produce
an unexpected service for a receiver that specified explicit
selection. As a result of these prohibitions, WF, SE, and FF
styles are all mutually incompatible.
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It would seem possible to simulate the effect of a WF reservation
using the SE style. When an application asked for WF, the RSVP
process on the receiver host could use local state to create an
equivalent SE reservation that explicitly listed all senders.
However, an SE reservation forces the packet classifier in each
node to explicitly select each sender in the list, while a WF
allows the packet classifier to simply "wild card" the sender
address and port. When there is a large list of senders, a WF
style reservation can therefore result in considerably less
overhead than an equivalent SE style reservation. For this
reason, both SE and WF are included in the protocol.
Other reservation options and styles may be defined in the future.
1.4 Examples of Styles
This section presents examples of each of the reservation styles
and shows the effects of merging.
Figure 4 illustrates a router with two incoming interfaces,
labeled (a) and (b), through which flows will arrive, and two
outgoing interfaces, labeled (c) and (d), through which data will
be forwarded. This topology will be assumed in the examples that
follow. There are three upstream senders; packets from sender S1
(S2 and S3) arrive through previous hop (a) ((b), respectively).
There are also three downstream receivers; packets bound for R1
(R2 and R3) are routed via outgoing interface (c) ((d),
respectively). We furthermore assume that outgoing interface (d)
is connected to a broadcast LAN, i.e., that replication occurs in
the network; R2 and R3 are reached via different next hop routers
(not shown).
We must also specify the multicast routes within the node of
Figure 4. Assume first that data packets from each Si shown in
Figure 4 are routed to both outgoing interfaces. Under this
assumption, Figures 5, 6, and 7 illustrate Wildcard-Filter,
Fixed-Filter, and Shared-Explicit reservations, respectively.
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RFC 2205 RSVP September 1997
________________
(a)| | (c)
( S1 ) ---------->| |----------> ( R1 )
| Router | |
(b)| | (d) |---> ( R2 )
( S2,S3 ) ------->| |------|
|________________| |---> ( R3 )
|
Figure 4: Router Configuration
For simplicity, these examples show flowspecs as one-dimensional
multiples of some base resource quantity B. The "Receives" column
shows the RSVP reservation requests received over outgoing
interfaces (c) and (d), and the "Reserves" column shows the
resulting reservation state for each interface. The "Sends"
column shows the reservation requests that are sent upstream to
previous hops (a) and (b). In the "Reserves" column, each box
represents one reserved "pipe" on the outgoing link, with the
corresponding flow descriptor.
Figure 5, showing the WF style, illustrates two distinct
situations in which merging is required. (1) Each of the two next
hops on interface (d) results in a separate RSVP reservation
request, as shown; these two requests must be merged into the
effective flowspec, 3B, that is used to make the reservation on
interface (d). (2) The reservations on the interfaces (c) and (d)
must be merged in order to forward the reservation requests
upstream; as a result, the larger flowspec 4B is forwarded
upstream to each previous hop.
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RFC 2205 RSVP September 1997
|
Sends | Reserves Receives
|
| _______
WF( *{4B} )
RFC 2205 RSVP September 1997
Figure 7 shows an example of Shared-Explicit (SE) style
reservations. When SE-style reservations are merged, the
resulting filter spec is the union of the original filter specs,
and the resulting flowspec is the largest flowspec.
|
Sends | Reserves Receives
|
| ________
SE( S1{3B} )
RFC 2205 RSVP September 1997
_______________
(a)| | (c)
( S1 ) ---------->| >-----------> |----------> ( R1 )
| > |
| > |
(b)| > | (d)
( S2,S3 ) ------->| >-------->--> |----------> ( R2, R3 )
|_______________|
Router Configuration
|
Sends | Reserves Receives
|
| _______
WF( *{4B} )
RFC 2205 RSVP September 1997
2. RSVP Protocol Mechanisms
2.1 RSVP Messages
Previous Incoming Outgoing Next
Hops Interfaces Interfaces Hops
_____ _____________________ _____
| | data --> | | data --> | |
| A |-----------| a c |--------------| C |
|_____| Path --> | | Path --> |_____|
| | data --> | |_____|
|_____| |--------| b d |-----------|
| Path-->| | Path --> | _____
_____ |
RFC 2205 RSVP September 1997
Each RSVP sender host transmits RSVP "Path" messages downstream
along the uni-/multicast routes provided by the routing
protocol(s), following the paths of the data. These Path messages
store "path state" in each node along the way. This path state
includes at least the unicast IP address of the previous hop node,
which is used to route the Resv messages hop-by-hop in the reverse
direction. (In the future, some routing protocols may supply
reverse path forwarding information directly, replacing the
reverse-routing function of path state).
A Path message contains the following information in addition to
the previous hop address:
o Sender Template
A Path message is required to carry a Sender Template, which
describes the format of data packets that the sender will
originate. This template is in the form of a filter spec
that could be used to select this sender's packets from
others in the same session on the same link.
Sender Templates have exactly the same expressive power and
format as filter specs that appear in Resv messages.
Therefore a Sender Template may specify only the sender IP
address and optionally the UDP/TCP sender port, and it
assumes the protocol Id specified for the session.
o Sender Tspec
A Path message is required to carry a Sender Tspec, which
defines the traffic characteristics of the data flow that the
sender will generate. This Tspec is used by traffic control
to prevent over-reservation, and perhaps unnecessary
Admission Control failures.
o Adspec
A Path message may carry a package of OPWA advertising
information, known as an "Adspec". An Adspec received in a
Path message is passed to the local traffic control, which
returns an updated Adspec; the updated version is then
forwarded in Path messages sent downstream.
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Path messages are sent with the same source and destination
addresses as the data, so that they will be routed correctly
through non-RSVP clouds (see Section 2.9). On the other hand,
Resv messages are sent hop-by-hop; each RSVP-speaking node
forwards a Resv message to the unicast address of a previous RSVP
hop.
2.2 Merging Flowspecs
A Resv message forwarded to a previous hop carries a flowspec that
is the "largest" of the flowspecs requested by the next hops to
which the data flow will be sent (however, see Section 3.5 for a
different merging rule used in certain cases). We say the
flowspecs have been "merged". The examples shown in Section 1.4
illustrated another case of merging, when there are multiple
reservation requests from different next hops for the same session
and with the same filter spec, but RSVP should install only one
reservation on that interface. Here again, the installed
reservation should have an effective flowspec that is the
"largest" of the flowspecs requested by the different next hops.
Since flowspecs are opaque to RSVP, the actual rules for comparing
flowspecs must be defined and implemented outside RSVP proper.
The comparison rules are defined in the appropriate integrated
service specification document. An RSVP implementation will need
to call service-specific routines to perform flowspec merging.
Note that flowspecs are generally multi-dimensional vectors; they
may contain both Tspec and Rspec components, each of which may
itself be multi-dimensional. Therefore, it may not be possible to
strictly order two flowspecs. For example, if one request calls
for a higher bandwidth and another calls for a tighter delay
bound, one is not "larger" than the other. In such a case,
instead of taking the larger, the service-specific merging
routines must be able to return a third flowspec that is at least
as large as each; mathematically, this is the "least upper bound"
(LUB). In some cases, a flowspec at least as small is needed;
this is the "greatest lower bound" (GLB) GLB (Greatest Lower
Bound).
The following steps are used to calculate the effective flowspec
(Re, Te) to be installed on an interface [RFC 2210]. Here Te is
the effective Tspec and Re is the effective Rspec.
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1. An effective flowspec is determined for the outgoing
interface. Depending upon the link-layer technology, this
may require merging flowspecs from different next hops; this
means computing the effective flowspec as the LUB of the
flowspecs. Note that what flowspecs to merge is determined
by the link layer medium (see Section 3.11.2), while how to
merge them is determined by the service model in use [RFC
2210].
The result is a flowspec that is opaque to RSVP but actually
consists of the pair (Re, Resv_Te), where is Re is the
effective Rspec and Resv_Te is the effective Tspec.
2. A service-specific calculation of Path_Te, the sum of all
Tspecs that were supplied in Path messages from different
previous hops (e.g., some or all of A, B, and B' in Figure
9), is performed.
3. (Re, Resv_Te) and Path_Te are passed to traffic control.
Traffic control will compute the effective flowspec as the
"minimum" of Path_Te and Resv_Te, in a service-dependent
manner.
Section 3.11.6 defines a generic set of service-specific calls to
compare flowspecs, to compute the LUB and GLB of flowspecs, and to
compare and sum Tspecs.
2.3 Soft State
RSVP takes a "soft state" approach to managing the reservation
state in routers and hosts. RSVP soft state is created and
periodically refreshed by Path and Resv messages. The state is
deleted if no matching refresh messages arrive before the
expiration of a "cleanup timeout" interval. State may also be
deleted by an explicit "teardown" message, described in the next
section. At the expiration of each "refresh timeout" period and
after a state change, RSVP scans its state to build and forward
Path and Resv refresh messages to succeeding hops.
Path and Resv messages are idempotent. When a route changes, the
next Path message will initialize the path state on the new route,
and future Resv messages will establish reservation state there;
the state on the now-unused segment of the route will time out.
Thus, whether a message is "new" or a "refresh" is determined
separately at each node, depending upon the existence of state at
that node.
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RSVP sends its messages as IP datagrams with no reliability
enhancement. Periodic transmission of refresh messages by hosts
and routers is expected to handle the occasional loss of an RSVP
message. If the effective cleanup timeout is set to K times the
refresh timeout period, then RSVP can tolerate K-1 successive RSVP
packet losses without falsely deleting state. The network traffic
control mechanism should be statically configured to grant some
minimal bandwidth for RSVP messages to protect them from
congestion losses.
The state maintained by RSVP is dynamic; to change the set of
senders Si or to change any QoS request, a host simply starts
sending revised Path and/or Resv messages. The result will be an
appropriate adjustment in the RSVP state in all nodes along the
path; unused state will time out if it is not explicitly torn
down.
In steady state, state is refreshed hop-by-hop to allow merging.
When the received state differs from the stored state, the stored
state is updated. If this update results in modification of state
to be forwarded in refresh messages, these refresh messages must
be generated and forwarded immediately, so that state changes can
be propagated end-to-end without delay. However, propagation of a
change stops when and if it reaches a point where merging causes
no resulting state change. This minimizes RSVP control traffic
due to changes and is essential for scaling to large multicast
groups.
State that is received through a particular interface I* should
never be forwarded out the same interface. Conversely, state that
is forwarded out interface I* must be computed using only state
that arrived on interfaces different from I*. A trivial example
of this rule is illustrated in Figure 10, which shows a transit
router with one sender and one receiver on each interface (and
assumes one next/previous hop per interface). Interfaces (a) and
(c) serve as both outgoing and incoming interfaces for this
session. Both receivers are making wildcard-style reservations,
in which the Resv messages are forwarded to all previous hops for
senders in the group, with the exception of the next hop from
which they came. The result is independent reservations in the
two directions.
There is an additional rule governing the forwarding of Resv
messages: state from Resv messages received from outgoing
interface Io should be forwarded to incoming interface Ii only if
Path messages from Ii are forwarded to Io.
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________________
a | | c
( R1, S1 ) | Router | ( R2, S2 )
|________________|
Send | Receive
|
WF( *{3B}) (a) | (c) --> WF( *{4B})
|
Reserve on (a) | Reserve on (c)
__________ | __________
| * {4B} | | | * {3B} |
|__________| | |__________|
|
Figure 10: Independent Reservations
2.4 Teardown
RSVP "teardown" messages remove path or reservation state
immediately. Although it is not necessary to explicitly tear down
an old reservation, we recommend that all end hosts send a
teardown request as soon as an application finishes.
There are two types of RSVP teardown message, PathTear and
ResvTear. A PathTear message travels towards all receivers
downstream from its point of initiation and deletes path state, as
well as all dependent reservation state, along the way. An
ResvTear message deletes reservation state and travels towards all
senders upstream from its point of initiation. A PathTear
(ResvTear) message may be conceptualized as a reversed-sense Path
message (Resv message, respectively).
A teardown request may be initiated either by an application in an
end system (sender or receiver), or by a router as the result of
state timeout or service preemption. Once initiated, a teardown
request must be forwarded hop-by-hop without delay. A teardown
message deletes the specified state in the node where it is
received. As always, this state change will be propagated
immediately to the next node, but only if there will be a net
change after merging. As a result, a ResvTear message will prune
the reservation state back (only) as far as possible.
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Like all other RSVP messages, teardown requests are not delivered
reliably. The loss of a teardown request message will not cause a
protocol failure because the unused state will eventually time out
even though it is not explicitly deleted. If a teardown message
is lost, the router that failed to receive that message will time
out its state and initiate a new teardown message beyond the loss
point. Assuming that RSVP message loss probability is small, the
longest time to delete state will seldom exceed one refresh
timeout period.
It should be possible to tear down any subset of the established
state. For path state, the granularity for teardown is a single
sender. For reservation state, the granularity is an individual
filter spec. For example, refer to Figure 7. Receiver R1 could
send a ResvTear message for sender S2 only (or for any subset of
the filter spec list), leaving S1 in place.
A ResvTear message specifies the style and filters; any flowspec
is ignored. Whatever flowspec is in place will be removed if all
its filter specs are torn down.
2.5 Errors
There are two RSVP error messages, ResvErr and PathErr. PathErr
messages are very simple; they are simply sent upstream to the
sender that created the error, and they do not change path state
in the nodes though which they pass. There are only a few
possible causes of path errors.
However, there are a number of ways for a syntactically valid
reservation request to fail at some node along the path. A node
may also decide to preempt an established reservation. The
handling of ResvErr messages is somewhat complex (Section 3.5).
Since a request that fails may be the result of merging a number
of requests, a reservation error must be reported to all of the
responsible receivers. In addition, merging heterogeneous
requests creates a potential difficulty known as the "killer
reservation" problem, in which one request could deny service to
another. There are actually two killer-reservation problems.
1. The first killer reservation problem (KR-I) arises when there
is already a reservation Q0 in place. If another receiver
now makes a larger reservation Q1 > Q0, the result of merging
Q0 and Q1 may be rejected by admission control in some
upstream node. This must not deny service to Q0.
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The solution to this problem is simple: when admission
control fails for a reservation request, any existing
reservation is left in place.
2. The second killer reservation problem (KR-II) is the
converse: the receiver making a reservation Q1 is persistent
even though Admission Control is failing for Q1 in some node.
This must not prevent a different receiver from now
establishing a smaller reservation Q0 that would succeed if
not merged with Q1.
To solve this problem, a ResvErr message establishes
additional state, called "blockade state", in each node
through which it passes. Blockade state in a node modifies
the merging procedure to omit the offending flowspec (Q1 in
the example) from the merge, allowing a smaller request to be
forwarded and established. The Q1 reservation state is said
to be "blockaded". Detailed rules are presented in Section
3.5.
A reservation request that fails Admission Control creates
blockade state but is left in place in nodes downstream of the
failure point. It has been suggested that these reservations
downstream from the failure represent "wasted" reservations and
should be timed out if not actively deleted. However, the
downstream reservations are left in place, for the following
reasons:
o There are two possible reasons for a receiver persisting in a
failed reservation: (1) it is polling for resource
availability along the entire path, or (2) it wants to obtain
the desired QoS along as much of the path as possible.
Certainly in the second case, and perhaps in the first case,
the receiver will want to hold onto the reservations it has
made downstream from the failure.
o If these downstream reservations were not retained, the
responsiveness of RSVP to certain transient failures would be
impaired. For example, suppose a route "flaps" to an
alternate route that is congested, so an existing reservation
suddenly fails, then quickly recovers to the original route.
The blockade state in each downstream router must not remove
the state or prevent its immediate refresh.
o If we did not refresh the downstream reservations, they might
time out, to be restored every Tb seconds (where Tb is the
blockade state timeout interval). Such intermittent behavior
might be very distressing for users.
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2.6 Confirmation
To request a confirmation for its reservation request, a receiver
Rj includes in the Resv message a confirmation-request object
containing Rj's IP address. At each merge point, only the largest
flowspec and any accompanying confirmation-request object is
forwarded upstream. If the reservation request from Rj is equal
to or smaller than the reservation in place on a node, its Resv is
not forwarded further, and if the Resv included a confirmation-
request object, a ResvConf message is sent back to Rj. If the
confirmation request is forwarded, it is forwarded immediately,
and no more than once for each request.
This confirmation mechanism has the following consequences:
o A new reservation request with a flowspec larger than any in
place for a session will normally result in either a ResvErr
or a ResvConf message back to the receiver from each sender.
In this case, the ResvConf message will be an end-to-end
confirmation.
o The receipt of a ResvConf gives no guarantees. Assume the
first two reservation requests from receivers R1 and R2
arrive at the node where they are merged. R2, whose
reservation was the second to arrive at that node, may
receive a ResvConf from that node while R1's request has not
yet propagated all the way to a matching sender and may still
fail. Thus, R2 may receive a ResvConf although there is no
end-to-end reservation in place; furthermore, R2 may receive
a ResvConf followed by a ResvErr.
2.7 Policy Control
RSVP-mediated QoS requests allow particular user(s) to obtain
preferential access to network resources. To prevent abuse, some
form of back pressure will generally be required on users who make
reservations. For example, such back pressure may be accomplished
by administrative access policies, or it may depend upon some form
of user feedback such as real or virtual billing for the "cost" of
a reservation. In any case, reliable user identification and
selective admission will generally be needed when a reservation is
requested.
The term "policy control" is used for the mechanisms required to
support access policies and back pressure for RSVP reservations.
When a new reservation is requested, each node must answer two
questions: "Are enough resources available to meet this request?"
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and "Is this user allowed to make this reservation?" These two
decisions are termed the "admission control" decision and the
"policy control" decision, respectively, and both must be
favorable in order for RSVP to make a reservation. Different
administrative domains in the Internet may have different
reservation policies.
The input to policy control is referred to as "policy data", which
RSVP carries in POLICY_DATA objects. Policy data may include
credentials identifying users or user classes, account numbers,
limits, quotas, etc. Like flowspecs, policy data is opaque to
RSVP, which simply passes it to policy control when required.
Similarly, merging of policy data must be done by the policy
control mechanism rather than by RSVP itself. Note that the merge
points for policy data are likely to be at the boundaries of
administrative domains. It may therefore be necessary to carry
accumulated and unmerged policy data upstream through multiple
nodes before reaching one of these merge points.
Carrying user-provided policy data in Resv messages presents a
potential scaling problem. When a multicast group has a large
number of receivers, it will be impossible or undesirable to carry
all receivers' policy data upstream. The policy data will have to
be administratively merged at places near the receivers, to avoid
excessive policy data. Further discussion of these issues and an
example of a policy control scheme will be found in [PolArch96].
Specifications for the format of policy data objects and RSVP
processing rules for them are under development.
2.8 Security
RSVP raises the following security issues.
o Message integrity and node authentication
Corrupted or spoofed reservation requests could lead to theft
of service by unauthorized parties or to denial of service
caused by locking up network resources. RSVP protects
against such attacks with a hop-by-hop authentication
mechanism using an encrypted hash function. The mechanism is
supported by INTEGRITY objects that may appear in any RSVP
message. These objects use a keyed cryptographic digest
technique, which assumes that RSVP neighbors share a secret.
Although this mechanism is part of the base RSVP
specification, it is described in a companion document
[Baker96].
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Widespread use of the RSVP integrity mechanism will require
the availability of the long-sought key management and
distribution infrastructure for routers. Until that
infrastructure becomes available, manual key management will
be required to secure RSVP message integrity.
o User authentication
Policy control will depend upon positive authentication of
the user responsible for each reservation request. Policy
data may therefore include cryptographically protected user
certificates. Specification of such certificates is a future
issue.
Even without globally-verifiable user certificates, it may be
possible to provide practical user authentication in many
cases by establishing a chain of trust, using the hop-by-hop
INTEGRITY mechanism described earlier.
o Secure data streams
The first two security issues concerned RSVP's operation. A
third security issue concerns resource reservations for
secure data streams. In particular, the use of IPSEC (IP
Security) in the data stream poses a problem for RSVP: if
the transport and higher level headers are encrypted, RSVP's
generalized port numbers cannot be used to define a session
or a sender.
To solve this problem, an RSVP extension has been defined in
which the security association identifier (IPSEC SPI) plays a
role roughly equivalent to the generalized ports [RFC 2207].
2.9 Non-RSVP Clouds
It is impossible to deploy RSVP (or any new protocol) at the same
moment throughout the entire Internet. Furthermore, RSVP may
never be deployed everywhere. RSVP must therefore provide correct
protocol operation even when two RSVP-capable routers are joined
by an arbitrary "cloud" of non-RSVP routers. Of course, an
intermediate cloud that does not support RSVP is unable to perform
resource reservation. However, if such a cloud has sufficient
capacity, it may still provide useful realtime service.
RSVP is designed to operate correctly through such a non-RSVP
cloud. Both RSVP and non-RSVP routers forward Path messages
towards the destination address using their local uni-/multicast
routing table. Therefore, the routing of Path messages will be
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unaffected by non-RSVP routers in the path. When a Path message
traverses a non-RSVP cloud, it carries to the next RSVP-capable
node the IP address of the last RSVP-capable router before
entering the cloud. An Resv message is then forwarded directly to
the next RSVP-capable router on the path(s) back towards the
source.
Even though RSVP operates correctly through a non-RSVP cloud, the
non-RSVP-capable nodes will in general perturb the QoS provided to
a receiver. Therefore, RSVP passes a `NonRSVP' flag bit to the
local traffic control mechanism when there are non-RSVP-capable
hops in the path to a given sender. Traffic control combines this
flag bit with its own sources of information, and forwards the
composed information on integrated service capability along the
path to receivers using Adspecs [RFC 2210].
Some topologies of RSVP routers and non-RSVP routers can cause
Resv messages to arrive at the wrong RSVP-capable node, or to
arrive at the wrong interface of the correct node. An RSVP
process must be prepared to handle either situation. If the
destination address does not match any local interface and the
message is not a Path or PathTear, the message must be forwarded
without further processing by this node. To handle the wrong
interface case, a "Logical Interface Handle" (LIH) is used. The
previous hop information included in a Path message includes not
only the IP address of the previous node but also an LIH defining
the logical outgoing interface; both values are stored in the path
state. A Resv message arriving at the addressed node carries both
the IP address and the LIH of the correct outgoing interface, i.e,
the interface that should receive the requested reservation,
regardless of which interface it arrives on.
The LIH may also be useful when RSVP reservations are made over a
complex link layer, to map between IP layer and link layer flow
entities.
2.10 Host Model
Before a session can be created, the session identification
(DestAddress, ProtocolId [, DstPort]) must be assigned and
communicated to all the senders and receivers by some out-of-band
mechanism. When an RSVP session is being set up, the following
events happen at the end systems.
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H1 A receiver joins the multicast group specified by
DestAddress, using IGMP.
H2 A potential sender starts sending RSVP Path messages to the
DestAddress.
H3 A receiver application receives a Path message.
H4 A receiver starts sending appropriate Resv messages,
specifying the desired flow descriptors.
H5 A sender application receives a Resv message.
H6 A sender starts sending data packets.
There are several synchronization considerations.
o H1 and H2 may happen in either order.
o Suppose that a new sender starts sending data (H6) but there
are no multicast routes because no receivers have joined the
group (H1). Then the data will be dropped at some router
node (which node depends upon the routing protocol) until
receivers(s) appear.
o Suppose that a new sender starts sending Path messages (H2)
and data (H6) simultaneously, and there are receivers but no
Resv messages have reached the sender yet (e.g., because its
Path messages have not yet propagated to the receiver(s)).
Then the initial data may arrive at receivers without the
desired QoS. The sender could mitigate this problem by
awaiting arrival of the first Resv message (H5); however,
receivers that are farther away may not have reservations in
place yet.
o If a receiver starts sending Resv messages (H4) before
receiving any Path messages (H3), RSVP will return error
messages to the receiver.
The receiver may simply choose to ignore such error messages,
or it may avoid them by waiting for Path messages before
sending Resv messages.
A specific application program interface (API) for RSVP is not
defined in this protocol spec, as it may be host system dependent.
However, Section 3.11.1 discusses the general requirements and
outlines a generic interface.
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3. RSVP Functional Specification
3.1 RSVP Message Formats
An RSVP message consists of a common header, followed by a body
consisting of a variable number of variable-length, typed
"objects". The following subsections define the formats of the
common header, the standard object header, and each of the RSVP
message types.
For each RSVP message type, there is a set of rules for the
permissible choice of object types. These rules are specified
using Backus-Naur Form (BNF) augmented with square brackets
surrounding optional sub-sequences. The BNF implies an order for
the objects in a message. However, in many (but not all) cases,
object order makes no logical difference. An implementation
should create messages with the objects in the order shown here,
but accept the objects in any permissible order.
3.1.1 Common Header
0 1 2 3
+-------------+-------------+-------------+-------------+
| Vers | Flags| Msg Type | RSVP Checksum |
+-------------+-------------+-------------+-------------+
| Send_TTL | (Reserved) | RSVP Length |
+-------------+-------------+-------------+-------------+
The fields in the common header are as follows:
Vers: 4 bits
Protocol version number. This is version 1.
Flags: 4 bits
0x01-0x08: Reserved
No flag bits are defined yet.
Msg Type: 8 bits
1 = Path
2 = Resv
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3 = PathErr
4 = ResvErr
5 = PathTear
6 = ResvTear
7 = ResvConf
RSVP Checksum: 16 bits
The one's complement of the one's complement sum of the
message, with the checksum field replaced by zero for the
purpose of computing the checksum. An all-zero value
means that no checksum was transmitted.
Send_TTL: 8 bits
The IP TTL value with which the message was sent. See
Section 3.8.
RSVP Length: 16 bits
The total length of this RSVP message in bytes, including
the common header and the variable-length objects that
follow.
3.1.2 Object Formats
Every object consists of one or more 32-bit words with a one-
word header, with the following format:
0 1 2 3
+-------------+-------------+-------------+-------------+
| Length (bytes) | Class-Num | C-Type |
+-------------+-------------+-------------+-------------+
| |
// (Object contents) //
| |
+-------------+-------------+-------------+-------------+
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An object header has the following fields:
Length
A 16-bit field containing the total object length in
bytes. Must always be a multiple of 4, and at least 4.
Class-Num
Identifies the object class; values of this field are
defined in Appendix A. Each object class has a name,
which is always capitalized in this document. An RSVP
implementation must recognize the following classes:
NULL
A NULL object has a Class-Num of zero, and its C-Type
is ignored. Its length must be at least 4, but can
be any multiple of 4. A NULL object may appear
anywhere in a sequence of objects, and its contents
will be ignored by the receiver.
SESSION
Contains the IP destination address (DestAddress),
the IP protocol id, and some form of generalized
destination port, to define a specific session for
the other objects that follow. Required in every
RSVP message.
RSVP_HOP
Carries the IP address of the RSVP-capable node that
sent this message and a logical outgoing interface
handle (LIH; see Section 3.3). This document refers
to a RSVP_HOP object as a PHOP ("previous hop")
object for downstream messages or as a NHOP (" next
hop") object for upstream messages.
TIME_VALUES
Contains the value for the refresh period R used by
the creator of the message; see Section 3.7.
Required in every Path and Resv message.
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STYLE
Defines the reservation style plus style-specific
information that is not in FLOWSPEC or FILTER_SPEC
objects. Required in every Resv message.
FLOWSPEC
Defines a desired QoS, in a Resv message.
FILTER_SPEC
Defines a subset of session data packets that should
receive the desired QoS (specified by a FLOWSPEC
object), in a Resv message.
SENDER_TEMPLATE
Contains a sender IP address and perhaps some
additional demultiplexing information to identify a
sender. Required in a Path message.
SENDER_TSPEC
Defines the traffic characteristics of a sender's
data flow. Required in a Path message.
ADSPEC
Carries OPWA data, in a Path message.
ERROR_SPEC
Specifies an error in a PathErr, ResvErr, or a
confirmation in a ResvConf message.
POLICY_DATA
Carries information that will allow a local policy
module to decide whether an associated reservation is
administratively permitted. May appear in Path,
Resv, PathErr, or ResvErr message.
The use of POLICY_DATA objects is not fully specified
at this time; a future document will fill this gap.
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INTEGRITY
Carries cryptographic data to authenticate the
originating node and to verify the contents of this
RSVP message. The use of the INTEGRITY object is
described in [Baker96].
SCOPE
Carries an explicit list of sender hosts towards
which the information in the message is to be
forwarded. May appear in a Resv, ResvErr, or
ResvTear message. See Section 3.4.
RESV_CONFIRM
Carries the IP address of a receiver that requested a
confirmation. May appear in a Resv or ResvConf
message.
C-Type
Object type, unique within Class-Num. Values are defined
in Appendix A.
The maximum object content length is 65528 bytes. The Class-
Num and C-Type fields may be used together as a 16-bit number
to define a unique type for each object.
The high-order two bits of the Class-Num is used to determine
what action a node should take if it does not recognize the
Class-Num of an object; see Section 3.10.
3.1.3 Path Messages
Each sender host periodically sends a Path message for each
data flow it originates. It contains a SENDER_TEMPLATE object
defining the format of the data packets and a SENDER_TSPEC
object specifying the traffic characteristics of the flow.
Optionally, it may contain may be an ADSPEC object carrying
advertising (OPWA) data for the flow.
A Path message travels from a sender to receiver(s) along the
same path(s) used by the data packets. The IP source address
of a Path message must be an address of the sender it
describes, while the destination address must be the
DestAddress for the session. These addresses assure that the
message will be correctly routed through a non-RSVP cloud.
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RFC 2205 RSVP September 1997
The format of a Path message is as follows:
::= [ ]
[ ... ]
[ ]
::=
[ ]
If the INTEGRITY object is present, it must immediately follow
the common header. There are no other requirements on
transmission order, although the above order is recommended.
Any number of POLICY_DATA objects may appear.
The PHOP (i.e., RSVP_HOP) object of each Path message contains
the previous hop address, i.e., the IP address of the interface
through which the Path message was most recently sent. It also
carries a logical interface handle (LIH).
Each RSVP-capable node along the path(s) captures a Path
message and processes it to create path state for the sender
defined by the SENDER_TEMPLATE and SESSION objects. Any
POLICY_DATA, SENDER_TSPEC, and ADSPEC objects are also saved in
the path state. If an error is encountered while processing a
Path message, a PathErr message is sent to the originating
sender of the Path message. Path messages must satisfy the
rules on SrcPort and DstPort in Section 3.2.
Periodically, the RSVP process at a node scans the path state
to create new Path messages to forward towards the receiver(s).
Each message contains a sender descriptor defining one sender,
and carries the original sender's IP address as its IP source
address. Path messages eventually reach the applications on
all receivers; however, they are not looped back to a receiver
running in the same application process as the sender.
The RSVP process forwards Path messages and replicates them as
required by multicast sessions, using routing information it
obtains from the appropriate uni-/multicast routing process.
The route depends upon the session DestAddress, and for some
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RFC 2205 RSVP September 1997
routing protocols also upon the source (sender's IP) address.
The routing information generally includes the list of zero or
more outgoing interfaces to which the Path message is to be
forwarded. Because each outgoing interface has a different IP
address, the Path messages sent out different interfaces
contain different PHOP addresses. In addition, ADSPEC objects
carried in Path messages will also generally differ for
different outgoing interfaces.
Path state for a given session and sender may not necessarily
have a unique PHOP or unique incoming interface. There are two
cases, corresponding to multicast and unicast sessions.
o Multicast Sessions
Multicast routing allows a stable distribution tree in
which Path messages from the same sender arrive from more
than one PHOP, and RSVP must be prepared to maintain all
such path state. The RSVP rules for handling this
situation are contained in Section 3.9. RSVP must not
forward (according to the rules of Section 3.9) Path
messages that arrive on an incoming interface different
from that provided by routing.
o Unicast Sessions
For a short period following a unicast route change
upstream, a node may receive Path messages from multiple
PHOPs for a given (session, sender) pair. The node cannot
reliably determine which is the right PHOP, although the
node will receive data from only one of the PHOPs at a
time. One implementation choice for RSVP is to ignore
PHOP in matching unicast past state, and allow the PHOP to
flip among the candidates. Another implementation choice
is to maintain path state for each PHOP and to send Resv
messages upstream towards all such PHOPs. In either case,
the situation is a transient; the unused path state will
time out or be torn down (because upstream path state
timed out).
3.1.4 Resv Messages
Resv messages carry reservation requests hop-by-hop from
receivers to senders, along the reverse paths of data flows for
the session. The IP destination address of a Resv message is
the unicast address of a previous-hop node, obtained from the
path state. The IP source address is an address of the node
that sent the message.
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The Resv message format is as follows:
::= [ ]
[ ] [ ]
[ ... ]